Protein Kinases are involved in very complex signaling cascades that regulate most cellular functions, including cell survival and proliferation. These signaling pathways have been heavily studied, particularly in the context of disorders caused by dysregulated cellular function, such as cancer. The mitogen-activated protein kinase (MAPK) cascade has been studied extensively, for example, and kinases in this pathway (e.g., RAS, RAF, MEK, and ERK) have been exploited as target sites for drug discovery. Mutated B-Raf is found in a significant fraction of malignancies (over 30% of all tumors and 40% of melanomas), and several drug candidates that inhibit a common B-Raf mutant (V600E, an activating mutation found in many cancers, particularly in cutaneous malignant melanoma, thyroid cancer, colorectal cancer, and ovarian cancer) have been reported, including GDC-0879, PLX4032, and PLX4720, while other inhibitors targeting C-Raf or B-Raf (or both) include sorafenib, XL281 RAF265, and BAY43-9006. These examples demonstrate that compounds that inhibit B-Raf or C-Raf are useful to treat various cancers.
The MAPK signaling cascade includes RAS, Raf, MEK and ERK kinases, each of which is actually a group of related proteins. Because they function collectively as a signal transduction cascade, the number of distinct kinases and their varying substrate specificities create a complex and highly branched pathway. Roskoski, Biochem. Biophys. Res. Comm., 399, 313-17 (2010). Raf, for example, consists of monomers referred to as A-Raf, B-Raf, and C-Raf (also called Raf-1), each of which functions primarily as a dimer. The RAF complex includes heterodimers as well as homodimers of these three species, bringing the total number of dimeric species in the Raf group to six, and each of these has a number of sites where phosphorylation at serine, threonine or tyrosine can cause either activation or inhibition. Matallanas, et al., Genes and Cancer 2:232 (2011, published online 10 May 2011). Due to the complexity of the pathway and its regulation, it has been reported that inhibitors of B-Raf can cause paradoxical activation of the pathway, apparently due to conformational effects on the kinase domain of Raf that affect dimerization, membrane localization, and interaction with RAS-GTP. Hatzivassiliou, et al., Nature, vol. 464, 431-36 (18 Mar. 2010). In particular, ATP-competitive inhibitors can exhibit opposing effects on the signaling pathway, as either inhibitors or activators, depending on the cellular context. As a result, B-Raf inhibitors effective against tumors having the activating B-Raf mutation V600E may not be as effective as expected in tumors having wild-type B-Raf or KRas mutations. Id.